Three port e-plane waveguide circulator



Jan. 27, 1970 MASAHIRO OMORI THREE PORT E-PLANE WAVEGUIDE CIRCULATOR Filed Dec. 21. 1967 FREQUENCY FIG. 36

INVENZOR M OMOR/ By 4 ATTORNEY United States Patent M US. Cl. 333--1.1 9 Claims ABSTRACT OF THE DISCLOSURE A three port E-plane waveguide Y-junction circulator in which the H-plane dimension of the junction cavity is restricted to substantially less than the H-plane dimension of the waveguides which form the junction so that the cavity is resonant in a transmission mode rather than in the absorption mode which appears to exist in the absence of restriction.

BACKGROUND OF THE INVENTION This invention relates to three port electromagnetic wave, waveguide circulators of the type in which three rectangular waveguides are joined in a Y-junction in a common E-plane and a magnetically polarized member of material having gyromagnetic properties is disposed in the cavity thus formed at the junction.

Recent examples of circulators of this type are described by G. Buchta in Miniaturized Broadband ETee Circulator at X-Band 54 proceedings of the IEEE 1607, November 1966; L. E. Davis and S. R. Longley, E- Plane 3-Port X-Band Waveguide Circulators, 11 IEEE Transactions on Microwave Theory and Techniques 443, September 1963; and J. W. McGowan and W. H. Wright, A High Power, Y-Junction, E-Plane Circulator, G-MT&T International Microwave Symposium Digest 85, 1967. It is recognized by this art that E-plane circulators have more compactness and substantial higher power capabilities than the more conventional H-plane circulators. These advantages make the E-plane form potentially useful for duplexer operations. However as further pointed out by this literature, the E-plane form suffers from narrow bandwidth, high losses and unexplained resonance peaks in the band of interest which have limited their actual usefulness to laboratory experiments.

SUMMARY OF THE INVENTION Analysis leading to the present invention demonstrates that the defects of previously proposed E-plane circulators stem from the junction cavity itself which seems to act as a parallel resonant circuit connected in series with the transmission path through the circulator. Since this resonance falls near or within the band of operation of the circulator, it absorbs substantial power, and introduces the observed narrow bandwidth and large insertion losses. In accordance with the present invention a novel cavity design is employed at the junction which converts the resonance into one equivalent to a parallel resonant circuit in parallel with the transmission path so that the cavity becomes a transmission cavity as opposed to the absorption cavity of the prior art. In particular the cavity has an H-plane dimension much smaller than the H-plane dimension of the joined guides which are in turn matched to the cavity by E-plane step transformers. According to a particular design the cavity is cylindrical and its H-plane dimension is readily adjustable over a broad range by at least one and possibly two screw plungers, the faces of which comprise the end boundaries of the cavity.

3,492,601 Patented Jan. 27, 1970 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cutaway perspective view of a waveguide circulator in accordance with the invention;

FIG. 2 is a cross-sectional view of a portion of FIG. 1; and

FIGS. 3A through 3D are typical transmission versus frequency characteristics representative, respectively, of various adjustments of the circulator in accordance with FIGS. 1 and 2.

DETAILED DESCRIPTION Referring more particularly to FIG. 1 and to the crosssectional view of a portion thereof in FIG. 2, three rectangular waveguides 11, 12 and 13 of conventional crosssectional dimensions are brought together to form a three port E-plane Y-junction which, according to accepted nomenclature, mean-s that the longitudinal axis of all guides intersect at equal angles and lie in a common plane which is parallel to the electric vector in the guides (parallel to the narrow wall). A conductively bounded cylinder 14 forms a cavity which is located at the intersection of the axes with the cylindrical axis thereof perpendicular to the common E-plane. Equally spaced rectangular apertures 15, 16 and 17 around the circumference of cylinder 14 connect respectively to each of guides 11, 12 and 13. In accordance with a preferred form of the invention apertures 15, 16 and 17 have H-plane dimensions (parallel to the wide wall and designated a on FIG. 2) which are equal to the I-I-plane dimension of guides 11, 12 and 13 and E-plane dimensions (designated b on FIG. 2) which are in the order of one-half the E- plane dimension b of guides 11, 12 and 13. Transition from the cross section of apertures 15, 16 and 17 to that of the guides 11, 12 and 13 is made by steps such as 21 or 22 in one or both of the wide walls of each guide. While these steps act as one-quarter wave transformers of conventional design, the physical distance between each step and the conductive wall of cavity 14 is less than any odd multiple of one-quarter guide wavelengths at the operating frequency in order to accommodate the susceptance of the cavity as will be described. It should be understood that other arrangements known in the art to be equivalent to step transformers may be employed in the transition.

In accordance with the invention the H-plane dimension of cavity 14 is shortened at one or both of its ends by a conductive boundary. According to the particular design shown in FIG. 1, ready adjustment of the effective cavity length (a-h) is facilitated by employing conductive screw plungers 18 and 19 which each cooperate with threads upon the inner surface of cavity 14 such that either plunger may be positioned at will along the cavity axis to close a portion h thereof. It should be understood however that the cavity length may be determined in other ways, either fixed or adjustable. For normal adjustment one piston such as piston 18 alone is suflicient and for this reason piston 19 is shown with its face in line with the top boundary of apertures 15, 16 and 17. The circulator is completed in usual fashion by suitably bonding to the top face of plunger 18 a disk or cylinder 20 of material exhibiting gyromagnetic properties, such as a substantially nonconductive ferrimagnetic or ferromagnetic material, for example, yttrium iron garnet or ferrite. Cylinder 20 preferably has both a diameter and an axial length somewhat less than b although this relationship is a matter of design and does not appear to be critical. In accordance with normal practice, means not shown are provided for supplying a steady magnetic biasing field through cylinder 20 in a direction parallel to the axis of the cylinder as represented by the vector H Operation of the circulator can be understood on the basis of the analysis given by Fay and Comstock in Operation of the Ferrite Junction circulator 13 IEEE Transactions on Microwave Theory and Techniques 15, January 1965. Thus excitation at any one of guides 11, 12 or 13 sets up a pair of counter rotating field patterns of the TM mode in cylinder acting as a dielectric resonator when a resonance of the cavity is near the frequency of excitation. The oppositely rotating modes have phases different from each other by an amount dependent upon the value of H Electric field nodal points are produced in the electrical field component perpendicular to the broad faces of guides 11, 12 and 13 when the two rotating fields are of opposite phase at a point. For the proper value of H a nodal point will be formed at the position of one port adjacent to the excited port while the remaining port will be remote from a nodal point. The coupling is nonreciprocal to the extent that excitation of a different port will set up a different pattern of node and antinode points. For further consideration and a detailed mathematical analysis of the phenomenon described, reference may be had to the publication of Fay and Comstock.

The bandwith and loss determining resonances, both of the prior art and of the present invention, as well as the proper adjustment in accordance with the invention, may be observed by removing the several transformers such as 21 and 22 and directly exciting any one of the cavity 14 ports at the desired operating frequency. Removing the transformers produces such a mismatch at the junction that circulator action is destroyed and the junction becomes primarily a reciprocal transmission structure having interposed therein a TM resonator. FIGS. 3A through 3D represent transmission versus frequency characteristics through such a transmission structure for various significant positions of piston 18. Thus FIG. 3A represents the transmission versus frequency characteristic through the junction when the surfaces of both piston 18 and 19 are flush with the top and bottom respectively of apertures 15, 16 and 17 so that h as shown on FIG. 2 is zero and the cavity has a total length equal to a. The resonator exhibits the sharp dip or loss peak 31 at a frequency in the band which is characteristic of a resonant circuit in series with the transmission path. It is believed that this peak is the characteristic responsible for observed operating characteristics in all known prior art E-plane circulators. As piston 18 is inserted to increase the distance h, the ratio of h to a increases and the loss peak 31 moves to higher frequencies. When It approximately equals 0.3a, a transition region occurs in which the characteristic seems to exhibit a loss peak such as 32 superimposed upon a broadband characteristic 33 typical of a parallel tuned circuit in parallel with the transmission path. This characteristic is shown dotted in FIG. 3B since its exact nature is difficult to observe with certainty. As 12 is further increased, a clearly defined broadband pass characteristic 34 as shown in FIG. 3C develops representing tight coupling and good transmission through the junction with low loss over a broadband. I specific embodiments this region appeared to have suitable characteristic in the range of 11 approximately 035a to 055a with apparent optimum with h approximately equal to 0.4a. Increasing h further into the range of 11 approximately equal to 0.6a, the bandpass characteristic shifts into that of a sharply resonant peak 35 of a parallel circuit in shunt across the transmission channel as shown in FIG. 3D. In accordance with the present invention, therefore, operation is adjusted to occur in the range between the transition region of FIG. 3B and the sharply tuned region of FIG. 3D. Based upon the ratios noted above this means that the H-plane dimension of cavity 14 that remains open is in the range from 0.65 to 0.45 of the H-plane dimensions of guides 11, 12 and 13.

Having thus determined the proper H-plane dimension of cavity 14 the transformers such as 21 and 22 are added to each port and fine adjustment of the impedance match between the cavity susceptance and the impedance of each of the waveguides 11, 12 and 13 is made by small adjustments of one or both of pistons 18 and 19. With proper design piston 19 may be eliminated and piston 18 may be fixed to a value determined experimentally for a model built in accordance with FIG. 1.

In a particular experimental model designed for operation in the 4.5 gHz. range and having proportions as specified above, 30 db isolation was observed over a band of more than 10 percent with an insertion loss of 0.1 db or 20 db isolation over a band of more than 20 percent with an insertion loss of less than 0.2 db.

It should be understood that the cylindrical design of cavity 14 is preferred only for its simplicity of design and its lack of sharp edges which might produce high voltage breakdowns. Other cavity cross sections, such as square and triangular, heretofore used in the art may be employed. Furthermore other forms of cavity closures may be used provided they shorten the cavity for all modes in which it is capable of operation and effectively close the appropriate fraction h of the H-plane dimension of each of apertures 15, 16 and 17. Note however that an axially extending conductive proble which does not fill the cavity, allows circular magnetic modes to be supported in the lower portion of the cavity and does not completely close the apertures has been determined experimentally to be unsatisfactory.

In all cases it is to be understood that the abovedescribed arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electromagnetic waveguide circulator comprising a plurality of rectangular waveguides brought together in an E-plane junction, a magnetically polarized member of gyromagnetic material disposed in the portion of said junction common to said guides, and conductive means for closing a substantial portion of the I-I-plane cross section of each of said guides at said junction.

2. The circulator according to claim 1 wherein said conductive means foreshortens the full H-plane dimen sion of said common portion.

3. The circulator according to claim 1 wherein said conductive means tunes said common portion to a point between a resonant condition having an absorption characteristic and a resonant condition having a transmission characteristic.

4. The circulator according to claim 1 including means in each of said guides for matching the impedance of said guides to the impedance of said common portion.

5. The circulator according to claim 4 wherein said means for matching comprises a step transformer in each of said guides.

6. The circulator according to claim 1 wherein said common portion is cylindrically shaped.

7. The circulator according to claim 6 wherein sa d conductive means comprises at least one cylindrically shaped plunger which fits within said cylindrically shaped common portion and is adapted for axial positioning therein.

8. An electromagnetic waveguide circulator of the type in which a magnetically polarized member of gyromagnetic material is disposed in an E-plane Y-junction of a plurality of conductively bounded rectangular waveguides, characterized in that the conductively bounded cross section of said junction is substantially uniformly less in its H-plane dimension than the H-plane dimension of the cross section of each of said guides.

9. The circulator according to claim 8 wherein said H-plane dimension of said junction is in the order of 0.65 to 0.45 of the H-plane dimension of said guides.

(References on following page) 5 6 References Cited HERMAN KARL SAALBACH, Primary Examiner UNITED STATES PATENTS P. L. GENSLER, Assistant Examiner 3,015,787 1/1962 Allin et a1. 333-11 s CL 3,085,212 4/1963 Clark et a1. 3331.1 333-33 3,311,849 3/1967 Bosma 333-1.1 5 

